Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Numerator:
Net income attributable to Biogen Inc.	$ 387.9	$ 303.8
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	144,400	147,100
Effect of dilutive securities:
Dilutive potential common shares	800	500
Shares used in calculating diluted earnings per share	145,200	147,600
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	600	300
Market stock units
Effect of dilutive securities:
Stock units	100	100
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100